the Lojban word for 'date' is detri. it also has a preposition to go along with it, de'i. it seems quite reasonable that a word used to indicate the date when something happens would be a preposition, right? after all, it lets you say things like de'i li 2025 for 'in 2025', or de'i li 27-9-2026 for the day I wrote this article. or should it be de'i li 2026-9-27? at least here the year can show you unambiguously if the date is small to large or large to small, but what if I want to say "on January 2nd", and leave the year to context? or, even worse, what if I want to say "since January 2nd"? de'i won't work, because you can't say co'a de'i li 1-2 (or 2-1). two prepositions one after the other don't modify each other; this means co'a ku de'i li 1-2 — which, if you interpret the aspects to have scope1, has the same meaning anyway, but surely it should be simpler to use a date as a noun form. "every 15th [of the month]" is very hard to say with de'i, and so is "February 13th was a bad day".
there is a way to say this with existing words, but it's long and quite clunky:
lo se detri be li 2-13 cu mabla djedi
in fact, I don't think I've ever seen anyone do this. most of the time, the preposition for dates ends up being so clunky that people resort to tanru like lo relma'i 13 moi (leaving the fact that it's a day to context, because here it's quite clear it is one) instead of doing it the "proper" way (if there even is a "proper" way).
well, what if de'i wasn't a preposition to begin with? it could be in LA'E, and turn a numerical noun phrase like li dau pi'e reno '10-20' — or better, me'o dau pi'e reno, because a date isn't really just a single number — into de'i me'o dau pi'e reno 'October 20th'. this would make it far easier to use dates as the argument of a verb — "February 13th was a bad day" just becomes de'i me'o 2-13 cu mabla djedi, and you can finally pass dates as arguments to prepositions without having to say lo se detri be...
or — even better — we could have de'i just be in LI2 instead, since you're always going to have the word me'o after it. de'i pasobiso is 'the year 1989', no li or me'o required. if you want to use this to say when an event happened (like CLL «de'i li 1989»), well, just use ca, as you would for any other noun form. here's some examples of how this new de'i could work:
ca de'i 1989 la .berlin. bitmu cu se daspo
'the Berlin Wall fell in 1989.'
mi jbena ca de'i 2006
'I was born in 2006.'
co'a de'i 2011 ba'o kulmulbi'o fa 40 tadni
'since 2011, forty students have graduated.'
‹na'a›
there's still a problem we haven't solved, though — sometimes you want to say a date that's just a day, as in "I'm going to France on the 17th". our de'i 17 just doesn't cut it — most of the time, context is going to tell you we're not talking about dates in the first century, sure, but sometimes people clip years, and "17" actually means "2017". so we're probably better off marking days and years with different words.
de'i comes from detri, but it sounds a lot like djedi. so let's make de'i mark specifically days. if I say de'i 17, you know I mean "on the 17th" (of whichever month is relevant), while if I want to say "in the year 17" (either the actual year 17, or short for 2017, 1917...), I'll use another word, na'a3, from nanca. so now we can tell de'i paze and na'a paze4 apart. so, the three example sentences from above would actually look like this:
ca na'a 1989 la .berlin. bitmu cu se daspo
mi jbena ca na'a 2006
co'a na'a 2011 ba'o kulmulbi'o fa 40 tadni
while anything that involves a day would look like this:
mi ba klama le frasygu'e ca de'i 17
'I'm going to France on the 17th.'
there's also another plus to having two different words: they naturally extend both ways. if you can say de'i 17, you can say de'i 17-9 for 'the 17th of September', and de'i 17-9-2027 for '17th September 2027'. so you can have de'i DAY, de'i DAY pi'e MONTH, and even de'i DAY pi'e MONTH pi'e YEAR. na'a, on the other hand, begins with the year, so it extends the other way: na'a 1960, na'a 1960-7 'July 1960', and for a full large-to-small date, na'a 1960-7-23.
so, here's another thing my proposal solves: full dates in Lojban can go either large to small or small to large, depending on the specific word you choose to tag them.
(the only combination we haven't covered here is the case where you only have a month, but that's already covered by the month names — pavma'i, relma'i... duvma'i, felma'i, gamyma'i. also, you can always mix the new words with month verbs; de'i 20 pe lo pavma'i means the same as de'i 20-1, just like lo pavma'i be ca na'a 2030 means 'January 2030'.)
oh, times of day work the same too
the preposition that marks times of day in official Lojban is ti'u. in my proposal, we keep the word, but just like de'i and the new na'a, we put it in the LI series. the number after it is hours-minutes-seconds (but this is agreed upon, unlike the whole problem with date formats). so
mi nitcu lo ka zvati lo jajysfi ca ti'u 16
'I need to be at this party at 4 o'clock.'
(24-hour time is preferred in Lojban, even colloquially.)
conclusion
so, with just a few changes and the introduction of one completely new word (na'a), I think we've managed to make Lojban's date system way more usable. I hope you like my proposal ^_^ it has already been adopted by one or two people since I first proposed it!